What he said.
Of note, the ogive of the bullet has to be shaped so that it doesn’t interfere with the internals of the GI Mag. Basically it has to follow the same basic geometry as a 5.56 round loaded with a nominal COAL. You may find that some 30 Cal bullets won work well for 300 BLK even when following SAAMI COAL.
Before you load up 100 rounds with a new bullet I’d load ten to make sure they fit and feed from the magazine. I’ve been bit by this before.
Reloading...it’s like knitting for men.